There’s been a lot of talk in the last decade about humans returning to the moon and visiting Mars and establishing habitats where astronauts could live and work. But building a base on a harsh, dusty world with no oxygen and little protection from cosmic radiation is easier said than done. One of the best materials for doing that is a very Earthy material, concrete. Now, reports NASA, astronauts aboard the International Space Station (ISS) have experimented with mixing cement, a key ingredient in concrete, out of Earth’s gravity for the first time to understand how the material hardens under microgravity.
Concrete is a mixture of sand, gravel and rock that is combined with a slurry made from cement powder and water to create a stone-like building material. The cement, when mixed with water, forms a crystallized microstructure that binds everything together as it dries. It’s been a staple in the construction industry for millennia due to its durability, and it turns out it’s also well-suited to life on Mars.
“On missions to the Moon and Mars, humans and equipment will need to be protected from extreme temperatures and radiation, and the only way to do that is by building infrastructures on these extraterrestrial environments,” Aleksandra Radlinska, Penn State civil engineer and principal investigator of the new study in the journal Frontiers in Materials, says. “One idea is building with a concrete-like material in space. Concrete is very sturdy and provides better protection than many materials.”
The other big advantage of concrete is that future Martians wouldn’t have to lug around all their building materials with them; they could make concrete by mixing cement with rocks and dust on Mars, or moon dust, also known as lunar regolith. That is, if the cement behaves properly in low gravity.
To assess how cement works in space, ISS astronauts conducted an experiment called Microgravity Investigation of Cement Solidification (MICS). On May 21, 2018, an Antares rocket blasted a Cygnus resupply capsule called OA-9 to the ISS. Aboard the craft was the MICS mission, which included eight kits containing 120 variations of cement powder and its additives.
Each sample included a burst pouch full of water. Astronauts ruptured the water pouch then massaged the liquid into the cement powder sample and mixed it with a spatula for 90 minutes. Then, for some samples, isopropyl alcohol was added to arrest the hydration of the cement. The samples were allowed to dry for various times, then sent back to Earth for analysis aboard a later ISS return mission.
According to the press release, the samples show some key differences from cement produced on Earth. The biggest was increased porosity, or the number of open spaces, in the micro-structure of the cement. Porosity can affect the strength of cement and subsequently concrete, but the team has not yet analyzed the strength of the astro-cement. They will do that later this year in a process that destroys the samples.
Radlinska tells Jake Parks at Discover magazine that the space samples were also more uniform in density than those produced on Earth. The two materials were opposite to one another; space cement is porous and uniform, while Earth cement is not uniform but contains fewer air bubbles.
The study shows, primarily, that concrete can be produced in microgravity and that it has an impact on how cement behaves. “We confirmed the hypothesis that this can be done,” Radlinska says. “Now we can take next steps to find binders that are specific for space and for variable levels of gravity, from zero [gravity] to Mars [gravity] and in between.”
But there is still a lot to learn before astronauts start pouring concrete space bunkers. “Even though concrete has been used for so long on Earth, we still don’t necessarily understand all the aspects of the hydration process. Now we know there are some differences between Earth- and space-based systems and we can examine those differences to see which ones are beneficial and which ones are detrimental to using this material in space,” she explains in the press release. “Also, the samples were in sealed pouches, so another question is whether they would have additional complexities in an open space environment.”
The team is already moving forward with designing moon concrete. Radlinska tells Parks that mixing the concrete with lunar regolith, which is very fine and jagged, could reduce the porosity of the cement, and they have begun experiments mixing cement with moon-like dust.
Another cement experiment called MVP Cell-05 was sent to the ISS in December 2018. In that mission, astronauts used a centrifuge to mimic gravity conditions on the moon and Mars as they hydrated concrete. The results of that mission, along with MICS, may even have impacts on terra firma. “What we find could lead to improvements in concrete both in space and on Earth,” Radlinska says in another NASA release. “Since cement is used extensively around the world, even a small improvement could have a tremendous impact.”